That is interesting as the norm for PPPoE is IPv6 Native not Passthrough, but obviously a special ‘feature’ with Turknet Broadband as you detailed above 
I will chat with ASUS and hopefully they can help and fix 
‘WAN >Internet Connection> WAN Connection Type set [PPPoE], please select IPv6 Connection type set [Native]’
https://www.asus.com/support/FAQ/113990
In a completely sane world, madness is the only freedom (J.G.Ballard).
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